The first step is understanding the wastewater.

Met-Chem typically evaluates:

• Average wastewater flow in GPM
• Maximum or peak flow
• Hours of operation per day
• Wastewater source
• Influent suspended solids
• Dissolved contaminants
• Wastewater chemistry
• pH
• Temperature
• Solids settling characteristics
• Chemical precipitation requirements
• Coagulant requirements
• Polymer requirements
• Required discharge quality
• Available floor space
• Downstream sludge handling

Two facilities operating at the same flow rate can require significantly different clarifiers.

A 100 GPM wastewater stream containing large, dense mineral solids behaves very differently from a 100 GPM wastewater stream containing light metal-hydroxide floc.

Flow starts the sizing process. The characteristics of the wastewater complete it.

Wastewater flow is generally expressed in gallons per minute:

Q = Flow Rate in GPM

Average flow is useful for understanding normal operation.

Peak flow is equally important.

A clarifier that performs well at average flow may experience solids carryover when flow temporarily increases.

Clarifier design should therefore consider both:

Average Design Flow

and

Peak Design Flow

EPA design guidance has long treated surface overflow rate and peak hydraulic loading as important clarifier sizing criteria because excessive upward flow can carry solids into the effluent.

For industrial systems, Met-Chem also considers whether the upstream treatment process provides equalization.

An equalization tank can reduce sudden hydraulic changes and allow wastewater to enter the treatment system at a more consistent rate.

 One of the fundamental clarifier sizing relationships is the surface overflow rate, sometimes called surface loading rate or hydraulic loading rate.

The basic relationship is:

Surface Overflow Rate = Flow ÷ Settling Area

or:

SOR = Q ÷ A

Where:

Q = wastewater flow

A = effective settling area

This relationship represents the upward hydraulic velocity imposed on the clarification area.

If water moves upward faster than the solids can settle downward, solids may be carried out with the clarifier effluent.

EPA guidance describes surface overflow rate as an important measure of clarifier hydraulic capacity.

For an inclined plate clarifier, the effective settling area is based on the horizontal projected area of the plates, not simply the footprint of the tank.

Inclined plates dramatically increase the amount of settling area that can be installed inside a compact clarifier.

The projected area of one inclined plate can be calculated as:

Projected Area = Plate Length × Plate Width × cos(θ)

Where:

θ = plate angle from horizontal

For multiple plates:

Total Projected Settling Area = Number of Plates × Plate Length × Plate Width × cos(θ)

Met-Chem commonly uses plates installed at approximately 60 degrees from horizontal.

At 60 degrees:

cos(60°) = 0.50

That means the horizontal projected area of a plate is approximately half of its actual physical area.

 Assume an inclined settling plate is:

4 feet wide

and

8 feet long

Physical plate area:

4 ft × 8 ft = 32 ft²

At a 60-degree angle:

Projected Area = 32 × cos(60°)

Projected Area = 32 × 0.50

Projected Area = 16 ft²

If the clarifier contains 40 comparable settling plates:

40 × 16 ft² = 640 ft² projected settling area

This illustrates why inclined plate clarifiers can provide substantial settling area within a relatively small tank.

Once a design surface loading rate has been selected, required settling area can be estimated using:

Required Area = Flow ÷ Design Surface Loading Rate

or:

A = Q ÷ SLR

For example:

Assume:

Flow = 100 GPM

Design loading = 0.25 GPM/ft²

Then:

A = 100 ÷ 0.25

A = 400 ft² of projected settling area

This does not automatically mean that every 100 GPM industrial wastewater application should use 400 ft².

The appropriate loading rate depends on the type of solids, settling characteristics, chemical treatment, required effluent quality, and other process conditions.

Published industrial inclined-plate guidance commonly shows that lighter metal-hydroxide solids require more conservative loading than dense, rapidly settling particles.

 Clarifier performance ultimately depends on the relationship between:

Upward water velocity

and

Downward solids settling velocity

For effective separation:

Solids settling velocity should exceed the upward hydraulic velocity through the clarification area.

If solids settle more slowly than the water rises, they can be carried upward and discharged with the clarified water.

This is why wastewater testing is valuable.

A simple settling test can provide information about:

• Floc formation
• Settling velocity
• Final sludge volume
• Water clarity
• Chemical dosage
• Polymer dosage

Where wastewater characteristics are unusual, laboratory or pilot testing can provide more reliable design information than using a generic loading value. EPA design guidance also recommends testing or special consideration where industrial wastewater characteristics differ significantly from typical wastewater.

Met-Chem commonly uses inclined settling plates positioned at approximately 60 degrees.

Plate angle affects both:

Projected settling area

and

the ability of accumulated sludge to slide down the plate.

A flatter plate provides more projected area.

However, if the plate becomes too shallow, sludge may not slide effectively.

For industrial wastewater, the plate angle must therefore balance hydraulic performance with reliable solids removal.

EPA literature on inclined plate settlers notes typical plate angles around 55–65 degrees, depending on the application.

 Projected settling area determines hydraulic settling capacity.

Tank volume determines hydraulic retention time.

Met-Chem considers both.

Hydraulic retention time can be estimated as:

Retention Time = Available Liquid Volume ÷ Flow Rate

For example:

If a clarifier contains approximately:

6,000 gallons

and receives:

100 GPM

then:

Retention Time = 6,000 ÷ 100

Retention Time = 60 minutes

Met-Chem standard industrial clarification systems are commonly designed around approximately 50–60 minutes of overall hydraulic retention time, depending on the configuration.

Retention time can provide:

• Hydraulic buffering
• Additional floc development
• Reduced flow disturbance
• Additional solids settling opportunity
• Sludge storage
• Better tolerance for changing wastewater conditions

For industrial wastewater, this can be particularly valuable because flow and chemistry are rarely perfectly constant throughout a production day.

Plate spacing is another important design consideration.

Reducing the distance between plates allows more plates to fit within a given tank.

More plates can theoretically provide additional settling area.

However, tighter plate spacing can also reduce the available channel width for:

• Wastewater flow
• Floc movement
• Sludge movement
• Cleaning
• Maintenance

Industrial wastewater often produces relatively bulky floc.

Examples include:

• Aluminum hydroxide
• Iron hydroxide
• Heavy-metal hydroxides
• Paint solids
• Mineral solids
• Chemical precipitates

The plate spacing must be large enough to prevent the plate pack from becoming excessively restricted or blinded by solids.

Published inclined-plate guidance shows that plate spacing can vary significantly based on solids characteristics and concentration.

More plates are not automatically better.

 It is possible to design a very small tank with a large amount of theoretical plate area.

That may produce an attractive GPM rating on paper.

However, industrial wastewater clarification requires more than theoretical area.

A clarifier also needs enough physical volume to accommodate:

• Flow fluctuations
• Flocculated solids
• Hydraulic distribution
• Chemical-treatment variations
• Sludge accumulation
• Sludge withdrawal
• Peak flow conditions

Met-Chem therefore evaluates projected settling area and tank retention time together.

A large plate pack inside an undersized tank is not necessarily a well-designed industrial clarifier.

 Consider two clarifiers that are both advertised for:

100 GPM

Provides sufficient inclined plate area but has very little tank volume.

At 100 GPM, the hydraulic retention time may only be 20–25 minutes.

Provides comparable projected plate area but has significantly more liquid volume.

At 100 GPM, the retention time may be approximately 50–60 minutes.

Both units may theoretically have enough plate area for the flow.

However, Clarifier B provides substantially more hydraulic buffering and process volume.

For industrial wastewater where solids concentration, chemical treatment, and flow can vary, Met-Chem prefers a conservative approach that considers the entire treatment process.